Films with high optical density (OD) of >3.5 or, respectively, very low transparency of <0.1% are needed by way of example for blinds in the greenhouse sector, where these blinds inhibit penetration of daylight and thus permit establishment of different day/night periods by using artificial light, irrespective of ambient light. These blinds also prevent impairment of the environment of the greenhouse by strong artificial light during the night. Another possible important use is found in the sector of roller blinds and Venetian blinds, where the intended use of these is for reduction of light levels. Films of this type are also suitable for use in projection screens and as films in display applications, for example for mobile telephones.
However, considerable further requirements are placed upon the film in particular by the first two application sectors (blinds, including roller blinds) and the last application sector (displays). By way of example, the thickness of these cannot be greater than 60 μm, otherwise they are insufficiently flexible (or quite simply too thick) for the final applications, and the film also has to have a certain UV resistance in order to be suitable for long-term use.
At least in the two first-mentioned applications, it is also desirable that the film has low flammability and ideally is self-extinguishing in the absence of any supportive fire, in order that it does not contribute to spread of fire in greenhouses and other buildings.
The films must also be white on both sides, in order to reflect a maximum portion of the incident light, for reasons which include avoidance of heating of the material itself and contribution to energy efficiency. High whiteness in reflected light or, respectively, high lightness in reflected light is also important.
The film is also intended to be capable of cost-effective production on existing multilayer polyester film plants. These generally comprise, in the thickness range up to 75 μm, a main extruder suitable for high throughputs, and from one to two extruders for the outer layers, suitable for markedly lower throughputs.
Films that are white on both sides and that have high optical density have been described by way of example in JP-A-63-220421 (Toray Ind. Inc.) or US-A-2007/0054141 (DuPont Films).
Although US-A-2007/0054141 gives a general description of films of thickness from 12 to 350 μm (preferably from 100 to 175 μm), the thicknesses of the films in the examples are all less than or equal to 100 μm. JP-A-63-220421, too, describes only films of thickness markedly above 100 μm, and also teaches that the thickness of the outer layers is to be at least 20 μm.
As the films become thicker, it becomes easier to achieve high whiteness of the outer layers (more difficult to perceive the black base layer through these) and it therefore also becomes easier to achieve high optical density (Lambert-Beers law). Neither of the specifications reveals a method for producing films with high whiteness and high optical density with total thickness markedly smaller than 100 μm. Nor is there any information about achievement of appropriate UV resistance and optional achievement of appropriate flame retardancy.